Mechanical advantage in the size-weight illusion

When a hand-held object is lifted by wrist flexion, the lifting system composed of muscles, bones, and the object lifted constitutes a third-class lever. Therefore, objects require greater lifting force as they are supported further distally The small can of the DeMoors size-weight illusion cans is usually supported further distally than the large one, possibly influencing their relative perceived weight When Ss are required to lift the small can through a shorter lever than the large one, there is a significant shift of judgments toward a reversal of the SWI in a paired-comparison situation. It is concluded that mechanical advantage does influence weight judgments and that biomechanical factors should be considered whenever weight judgments are made.